1. Field of the Invention
The present invention relates to an image forming optical system of a slim type, particularly suitable for an electronic image pickup optical system, which has a superior image forming performance, and an electronic image pickup apparatus using the same.
2. Description of the Related Art
A digital camera has reached a level of being used practically, regarding making large the number of pixels (high image quality), and small-sizing and slimming. Also from a function point of view and market point of view, a digital camera has substituted a silver salt 35 mm film camera. Therefore, as a next trend of evolution, further increase in the number of pixels and further slimming have been sought.
As a prior art with a potential for slimming, an optical system described in Japanese Patent Application Laid-open Publication No. 2005-266129 is available. In Japanese Patent Application Laid-open Publication No. 2005-266129, an image forming optical system which includes in order from an object side, a first lens group having a negative refracting power, a second lens group having a positive refracting power, and a third lens group having a positive refracting power has been proposed. Here, the first lens group includes one lens component in which, a positive lens and a negative lens are cemented in this order, and a cemented surface thereof is an aspheric surface. The second lens group includes a positive lens component and a negative lens component in this order. The third lens group includes one lens component. Moreover, in Japanese Patent Application Laid-open Publication No. 2007-156385, an image forming optical system which includes in order from an object side, a first lens group G1 having a negative refracting power, a second lens group G2 having a positive refracting power, a third lens group G3 having a positive refracting power, and a fourth lens group has been disclosed. Here, the first lens group G1 includes one negative lens having a biconcave shape, and the fourth lens group has an aspheric surface. Moreover, in this image forming optical system, zooming and focusing are carried out by changing distances between the lens groups.